Gemmological and Mineralogical Characteristic of A Kind of Blue-Green Seal Stone

The mineral components, chemical compositions and microstructure of a kind of blue-green seal stones newly emerging in the market(called "Tian Tsing Stone" in market) are analyzed using conventional gemmological tests, polarizing microscope observation, FTIR, EPMA and SEM tests. The results show that the main mineral component of the blue-green seal stone is kaolinite, and the minor minerals mainly include pyrite and thiophosphorus aluminum strontite. There is a bronzing layer on the surface of some samples, which is iron mineral. EPMA test shows that the main chemical compositions of the blue-green seal stone are SiO₂ and Al₂O₃, and the Cr element leads to a slightly blue-green tone in colour. The local red and yellow blocks in the samples are mainly caused by the iron content. The observation of SEM shows that the kaolinite minerals in the samples are mainly staggered stacks of scaly or pseudo-hexagonal plate-like crystals, and some are stacked disorderly or like books. Therefore the texture of the blue-green seal stone is fine and oily.